#include <triqs/gfs.hpp>
using namespace triqs::gfs;
int main() {
 double tmin = 0, tmax = 1.0;
 int nt = 100;
 auto g = gf<cartesian_product<retime, retime>, scalar_valued>{{{tmin, tmax, nt}, {tmin, tmax, nt}}};
 triqs::clef::placeholder<0> t1_;
 triqs::clef::placeholder<1> t2_;
 g(t1_, t2_) << 2 * t1_;
}